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Rotary Broaching Explained: How it Works

Rotary Broaching Explained: How it Works

Rotary Broaching Explained: How it Works

Rotary broaching is a machining process used for producing polygonal geometric features on a workpiece.

This operation has significant applications in the machining industry, due to its ability to produce unique features on workpieces that otherwise are difficult to machine.

But what exactly is rotary broaching and what makes it different from other machining operations?

Rotary broaching is a machining operation involving a polygon-shaped cutting tool that is mounted on a tool holder and fed against a rotating workpiece, either into a cavity, like an already drilled hole, or against the periphery of the workpiece to impart a polygon-shaped surface.

This article discusses rotary broaching in detail by going through its process and sheds some light on optimal parameters and advantages.

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Rotary Broaching Explained

Rotary broaching an already drilled hole
Rotary broaching an already drilled hole (Source: Gisstec)

Rotary broaching is a process of using a specialized tool to produce non-circular geometry on the workpiece.

Unlike boring, rotary broaching enlarges an already drilled hole by producing a specific geometry on the circumference of the hole.

Apart from that, rotary broaching can also be performed on the external surface of the workpiece to get a grooved pattern.

Generally, it involves a rotating workpiece and a cutting tool that come in contact to get the desired output.

The difference in the relative rotary speeds and friction between the two surfaces causes material removal, resulting in a polygon-shaped surface. 

This machining operation is usually performed on a lathe machine by using a rotary broach and a specialized tool holder.

Rotary Broach

It is a polygon-shaped tool, usually composed of high-speed steel, with the option of an additional coating of Titanium Nitride, Titanium Carbo-Nitride, or Titanium Aluminum-Nitride for enhanced durability.

A rotary broach can machine various materials like aluminum, brass, steel, plastic, etc.

It is fed against the workpiece at a 1° angle to its centerline during the broaching operation. 

The rotation of the broach is automatically synchronized with the rotation of the workpiece, creating a wobbly effect that vibrates the cutting tool inside the drilled hole and the cutting edge removes the material.

This machining process can impart a variety of surfaces on the workpiece, such as a hexagon, square,  keyway, letter shape, etc.

Tool manufacturers, such as Gisstec, produce broaches with different geometries, suitable for almost every broaching application.

Tool/Broach Holder

Rotary broach holder
Rotary broach clamped on a broach holder (Source: Gisstec)

The broach holder houses the broach and is attached to the tailstock of a lathe.

During operation, the lathe spindle rotates the workpiece, while the broach, clamped in the broach holder, rotates upon impact with the workpiece.

A free spinning bearing between the broach and broach holder ensures this mechanism.

Moreover, the broach holder holds the broach at an angle to the horizontal axis of the workpiece.

This feature leads to a wobbling motion due to which each broach tooth machines a groove and ultimately produces a polygon-shaped surface on the workpiece.

On the other hand, when performing broaching on a milling machine, the spindle rotates the broach holder while the workpiece and the broach are stationary in contact with each other.

The wobbling of the broach due to the impact leads to material removal and produces the desired profile.

Rotary Broaching- Steps Involved

Process of Rotary Broaching
Process of Rotary Broaching

Several steps need to be sequentially followed to obtain a clean and accurate broach.

Preparing to broach a hole 

Before drilling the actual hole, a chamfered hole is machined on the workpiece to guide the following drilling procedure.

This chamfer hole is slightly larger than the major dimension of the desired broach.

Drilling the Hole for Broaching

For internal broaching, drill a hole approximately 1% larger than the smallest broach dimension.

This difference will serve as a tolerance. However, different materials have different tolerances, and each broach has a specific set of tolerances stated in its respective catalog.

The drill hole should be at least 1.3 times longer than the broach length to allow space for chip accumulation.

Adjusting the Workpiece and Broach Centerline

Angular difference in the axis of the broach and the part to be broached
Angular difference in the axis of the broach and the part to be broached

Firstly, ensure that the workpiece is correctly centered and mounted on the machining center. Ensure that the chuck's jaws and screws are properly tightened.

After that, align the broach centerline to make an angle of about one degree or less with the workpiece centerline so that these two centerlines intersect at the point of contact between the broach and the workpiece surface. 

Executing the Broaching Operation

Finally, perform the broaching operation, but keep the feed rate optimized to avoid unwanted vibrations which can lead to tool and workpiece damage.

Use a water- or oil-based coolant as it ensures effective heat dissipation, increasing the tool's life. 

After broaching, remove any remaining chips from the inside of the broached surface in case of internal broaching.

Optimal Machining Parameters for Rotary Broaching

It is crucial to ensure optimal machining parameters for broaching to obtain the best results.

ParameterValues
Broach ShapeMore number of edges results in a smoother profile
Speed800 to 3,500 RPM
Feed Rateless than 0.03 times the hole profile diameter
Machining parameters for rotary broaching

Broach size/shape

You should choose the broach size depending on your application.

Large broaches have shanks with a diameter of around 8mm or more, whereas small broaches have diameters varying from 12.7 to 20 mm.

Broaches come in various shapes like squares, hexagons, octagons, etc., and the broach shape depends on the user's requirements.

Higher the number of edges of the broach, the higher the surface finish.

Recommended Speed 

Generally, a high speed of around 800 to 3,500 RPM is recommended for broaching.

The offset in the broach's angle facilitates the leading edge to actively remove the material even at high RPMs.

To enhance the tool life, the RPM should gradually be increased up to the set point to avoid unnecessary tool wear and tear. 

It also helps avoid unwanted vibrations that may lead to the generation of stresses within the workpiece and tool.

After achieving a broaching cycle, you can reverse the broach rotation and repeat the process to improve surface smoothness and accuracy.

Smaller broaches usually operate at 1000 to 3000 RPM, whereas 800 to 1,500 RPM is recommended for larger broaches.

Feed Rate for Broaching

The material type primarily dictates the feed rate choice.

Mild steel is a prevalent workpiece material, with an optimal feed rate between 0.0012 and 0.0024 inches per revolution. 

Generally, harder materials require slower feed rates, whereas softer materials like aluminum can operate on higher feed rates.

In some cases, a rule of thumb can be implemented which dictates that the feed rate should not exceed 0.03 times the hole profile diameter at any given instant. 

Generally, a slower feed rate results in a more accurate and smoother broached surface with finer lines, whereas a faster feed rate reduces the machining time but at the expense of surface smoothness.

Advantages of Rotary Broaching Over Other Methods

External rotary broaching on a workpiece
External rotary broaching on a workpiece (Source: Gisstec)

Various advantages render broaching useful in many industries, like the medical, automotive, oil and gas industries, etc.

It is commonly used to manufacture gears, sprockets, screws, and other fasteners.

 

Quick Machining Time

Rotary broaching is a quick and less time-consuming process compared to other similar methods, such as form punching.

This aspect makes it suitable for bulk production of goods, such as fasteners with high demand and requiring speedy continuous output.

Low Operational Cost

Rotary Broaching is also a cheap process, mainly due to the low maintenance cost and the required tools.

In some cases, due to being less time-consuming, it is even cheaper than other similar processes like form punching or stamping.

However, due to the high cost of the required machining centers, small-scale industries or hobbyists may not find it affordable.

Accessibility

Rotary broaching is achievable on various machines, ranging from vertical machining centers to horizontal machining centers.

Moreover, you can even use a drill press to perform this operation, enhancing its convenience.

High Accuracy

This machining operation, coupled with the optimal feed rate, applies significantly lesser force on the workpiece, resulting in lower stresses.

It ultimately leads to a more accurate and smooth broached surface that looks aesthetically better than a punched or stamped surface.

Generally, broaching provides high accuracy of around 0.0005″.

Factors to Consider in Rotary Broaching

Type of Coolant

An oil-based coolant is preferable as it enhances tool life by providing high lubricity between the workpiece and the broach.

On the other hand, a synthetic coolant provides better heat capacity at the cost of lubrication, rendering it unsuitable for broaching operations.

Broach Coating

The coating of the broach plays an important role in determining the surface finish of the finished product.

Generally, Titanium-Nickel coating is best suited for broaches, as it enhances its durability, tool life, and lubricity.

Machine Used for Broaching

Generally, broaching is performed on a lathe machine as it provides the ability to perfectly align the broach with the workpiece.

However, for broaching of asymmetrical workpieces where the broached geometry does not lie along the axis of rotation of the workpiece, a CNC router can be used to get the desired results.

Although a drill press can also be used for the operation, its low speed and accuracy limit it to DIY and small-scale projects only.

Final Thoughts

Rotary broaching is best suited for producing non-circular geometries on the workpiece, which otherwise require complex machining operations to produce.

It is a simple and fast process that can impart the required polygon design on the internal or external surface of the workpiece.

Depending upon the material of the broach, type of machine, and lubricant used, broaching can be performed on almost any material.

Frequently Asked Questions (FAQ)

How long does a broaching tool last?

Industrial-level broaching tools can machine around 10,000 parts before altogether wearing out; however, they must be reground and sharpened after every 3000 details. Small-scale broaching tools, often used by hobbyists and small-scale industries, last about 1000 parts, with sharpening required after every 200 to 300 workpieces.

Are rotary broaches available in custom sizes? 

Yes, rotary broaches are available in custom sizes. Different manufacturers prepare custom-sized broaches on order. However, ANSI (American National Standards Institute) rotary broach sizes are more readily available.

How many different types of broaching are there?

There are four types of rotary broaching, each differing based on tool orientation and machining principle. They are horizontal broaching machines, vertical broaching machines, surface broaching machines, and continuous broaching machines.

About John

Hey I'm John. I write about Manufacturing, Metalworking, CNCs and Lasers at Mellowpine. If you have any questions related to CNCs or Lasers, I'd be happy to answer them. Reach me at mail@mellowpine.com

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John

Hey I'm John. I write about Manufacturing, Metalworking, CNCs and Lasers at Mellowpine. If you have any questions related to CNCs or Lasers, I'd be happy to answer them. Reach me at mail@mellowpine.com

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